The European Union is warning Russia that interfering with gas deliveries to Ukraine to apply political pressure would backfire by damaging its reputation as a reliable supplier for the EU.

European Commission President Jose Manuel Barroso said in a letter on behalf of the EU’s 28 governments that Russia’s “contractual reliability as a supplier of gas is at stake in this matter.”

The letter to Russia’s President released Thursday was the bloc’s joint response to Vladimir Putin’s threat last week that Moscow might cut off gas supplies to Ukraine and the pipelines shipping gas to western Europe through Ukrainian territory if Kiev fails to pay its debts.